1. Field of the Invention
The present invention relates to a method of manufacturing a compound semiconductor substrate, and more specifically to a method of manufacturing a compound semiconductor substrate made of a gallium nitride substrate.
2. Description of the Related Art
There are great expectations for Gallium nitride (GaN) blue laser diodes as light sources of the next generation""s high-density disc systems. Techniques for reducing crystalline defects density using Epitaxial Lateral Overgrowth (ELO) in gallium nitride blue laser diode devices are disclosed in xe2x80x9cJapanese Examined Patent Application No. Hei 6-105797xe2x80x9d, xe2x80x9cJapanese Unexamined Patent Application No. Hei 10-312971xe2x80x9d, xe2x80x9cT. S. Zheleva et al., MRS Internet J. Nitride Semicond. Res. 4S1, G3.38 (1999)xe2x80x9d and so on. Further, it is disclosed in xe2x80x9cS. Nakamura et al., Proceedings of 2nd International Conference on Nitride Semiconductors, Tokushima (1997) p444xe2x80x9d and xe2x80x9cS. Nakamura et al., Jpn. J. Appl. Phys. 38 (1999) p226xe2x80x9d that a practical life-span of the gallium nitride blue laser diode device can be obtained through the use of these techniques.
However, there are still a great number of problems which have to be solved in order to manufacture gallium nitride blue laser diode devices in volume at lower prices through the use of the above ELO. Specifically, it is known that sapphire substrates used as crystal growth substrates cause degradation in laser performance due to low thermal conductivity and cleavage planes susceptible to failure in their formation, etc.
As one of effective solutions to the problems, it is proposed that electrically conductive gallium nitride substrates with low defects density are manufactured in volume, and laser structures are formed on the electrically conductive gallium nitride substrates. In xe2x80x9cA. Usui et al., Jpn. J. Appl. Phys. 36 (1997) L899.xe2x80x9d and xe2x80x9cJapanese Unexamined Patent Application No. Hei 10-312971xe2x80x9d, a method of manufacturing a gallium nitride substrate through the use of HVPE (Hydride Vapor Phase Epitaxial growth) is disclosed, and an attempt of forming a laser structure on the gallium nitride substrate is disclosed in xe2x80x9cM. Kuramoto et al., Jpn. J. Appl. Phys. 38 (1999) L184.xe2x80x9d In this case, the defects density in the gallium nitride substrate is about 107cmxe2x88x922, which is a higher value than the defects density of 106cmxe2x88x922 in a gallium nitride layer formed on a sapphire substrate through the use of ELO.
Further, after growing a thick-film gallium nitride layer on a sapphire substrate through the use of HVPE, there are no techniques established for easily removing the sapphire substrate from the thick-film gallium nitride layer, so the problem which has to be solved for mass production of the gallium nitride substrates still exists.
As one of the methods of removing the sapphire substrate from the gallium nitride layer, it is proposed that an excimer laser beam is applied to the back of the sapphire substrate to melt a substrate-nitride interface. However, it is difficult to remove the sapphire substrate in a diameter of 2 inches with high reproducibility. Moreover, a method of removing the sapphire substrate by polishing is an option, but the gallium nitride layer on the sapphire substrate has a large strain due to a difference in the thermal expansion coefficients between the gallium nitride layer and the sapphire substrate. Therefore, it is difficult to remove the sapphire substrate through the use of a typical polishing process.
As another method of manufacturing a gallium nitride substrate, a manufacturing method through the use of bulk growth under a high temperature and high pressure is disclosed in xe2x80x9cS. Porowski, Mat. Sci. and Eng. B44 (1997) 407xe2x80x9d. It is disclosed in xe2x80x9cS. Porowski et al., Mat. Res. Soc. Symp. Proc. 499 (1997) 35xe2x80x9d that a low defects density in a range from 103 cmxe2x88x922 to 105 cmxe2x88x922 can be achieved in this method. However, in this method, a practical crystal size required to form a laser diode device cannot be achieved.
In view of the forgoing, it is an object of the present invention to provide a method of manufacturing a compound semiconductor substrate.
A method of manufacturing a compound semiconductor substrate according to the invention comprises the steps of: forming a compound semiconductor layer on a single-crystal substrate through crystal growth so as to partially have a space between the compound semiconductor layer and the single-crystal substrate; and removing the compound semiconductor layer from the single-crystal substrate by irradiating the compound semiconductor layer from a side of the single-crystal substrate with a laser beam passing through the single-crystal substrate and being absorbed in the compound semiconductor layer to melt an interface between the single-crystal substrate and the compound semiconductor.
As a first method of forming the above compound semiconductor layer, a method of manufacturing a compound semiconductor substrate comprises the steps of: growing a first compound semiconductor layer on a single-crystal substrate through crystal growth; dividing the first compound semiconductor layer into a stripe shape, and forming a stripe-shaped depressed portion by removing an upper part of the single-crystal substrate below an interval of the first compound semiconductor layers divided into a stripe shape; forming a second compound semiconductor layer by growing mainly in a lateral direction from the first compound semiconductor layer having a stripe shape while maintaining a space between the second compound semiconductor layer and the single-crystal substrate; forming an insulating film or a high-melting point metal film on a portion of the second compound semiconductor layer grown on the top side of the first compound semiconductor layer and a meeting portion of the second compound layer; and forming a third compound semiconductor layer on the second compound semiconductor layer through crystal growth.
As a second method of forming the above compound semiconductor layer, a method of manufacturing a compound semiconductor substrate comprises the steps of: forming a stripe-shaped projected portion by processing an upper part of a single-crystal substrate; forming a first compound semiconductor layer through crystal growth mainly in a lateral direction on a top side of the stripe-shaped projected portion; forming an insulating film or a high-melting-point metal film on a portion of the first compound semiconductor layer grown on the top side of the stripe-shaped projected portion through crystal growth and a meeting portion of the first compound semiconductor layer; and forming a second compound semiconductor layer on the first compound semiconductor layer through crystal growth.
In the above method of a compound semiconductor substrate, after forming an insulating film or a high-melting point metal film on a portion of a compound semiconductor layer where a great number of crystalline defects exist, in the first method, the third compound semiconductor layer is formed, and in the second method, the second compound semiconductor layer is formed. Thereby, a thick-film compound semiconductor layer having lower defects density can be grown. After that, a laser beam passing through the single-crystal substrate and being absorbed in the first compound semiconductor layer is applied to the first compound semiconductor layer from a side of the single-crystal substrate to melt an interface between the single-crystal substrate and the first compound semiconductor layer, and thereby, the first compound semiconductor layer can be removed from the single-crystal substrate. Therefore, the single-crystal substrate can be removed without causing damage to the thick-film compound semiconductor layer (the third compound semiconductor layer in the first method, and the second compound semiconductor layer in the second method).
Thus, even a large strain occurs due to a difference in the thermal expansion coefficients, for example, between the gallium nitride layer and the sapphire substrate, the sapphire substrate can be easily removed. More specifically, as the first compound semiconductor layer is joined with the single-crystal substrate not in whole but in part, a space exists in a portion where the first compound semiconductor layer is not joined with the single-crystal substrate. Thereby, a laser beam is absorbed only in a portion where the single-crystal substrate and the compound semiconductor layer are joined with each other, so the single-crystal substrate can be easily removed.
Other and further objects, features and advantages of the invention will appear more fully from the following description.